1. Field of the Invention
The present invention relates to a titanium dioxide photocatalyst carrier having a photocatalytic activity and a process for its production.
2. Discussion of Background
In recent years, a research for hydrophilic, antibacterial or antifouling coating by titanium dioxide (titania, TiO2) has attracted an attention. Such a research is one utilizing a photocatalytic effect of titanium dioxide, and for example, a hydrophilic antifouling coat having titanium dioxide coated (JP-A-8-267646) and improvement of a photocatalytic activity by mixing it with a noble metal (JP-A-6-198196, JP-A-6-278241) have been reported.
On the other hand, a technique to form a thin layer of an oxide containing a titanium oxide as the main component, by carrying out a reactive direct current (DC) sputtering method in an oxidizing atmosphere by means of a metal target containing titanium as the main component, is common and has already been used for the production of heat reflecting glass, etc. However, the titanium dioxide layer obtainable by this method is amorphous from the viewpoint of X-rays and shows no substantial photocatalytic activity. Further, the sputtering method requires an expensive apparatus and is a method not suitable for mass production on an industrial scale.
As a method for forming a titanium dioxide layer showing a photocatalytic activity, a method of fixing fine particles of titanium dioxide by an organic or inorganic binder, or a metal organic decomposition (MOD) method which comprises coating and drying a metal organic solution to form an agglomerate of fine oxide powder and then subjecting it to heat treatment while supplying oxygen to form a -M-O-M-structure, has heretofore been known. Further, a sol (solution)-gel (solid) method is known wherein a metal alkoxide is dissolved in a suitable solvent such as an alcohol, this solution is subjected to hydrolytic condensation polymerization to prepare a polymer sol (colloidal dispersion), which is coated on a substrate, followed by drying and heat treatment for crystallization. This method has had a problem that it is difficult to store the dispersion in the sol state under a constant state for a long period of time, and a sol solution once used, can not be re-used, since the dispersion in the sol state is gelled in the solution to precipitate on a substrate for forming a layer, whereby a non-reacted product will also be gelled.
Fujishima Akira et al have reported in preparatory papers for the 6th symposium (at Tokyo University on Nov. 26, 1999) of Optically Functional Material Research Association, p. 158, that an alcohol dispersion of titanium oxide was formed into a layer by a sol-gel method on Nesa glass [which has a SnO2 (fluorine containing) layer on a glass], followed by baking at 500xc2x0 C. for 30 minutes. The titanium dioxide layer formed on the Nesa glass obtained by this method, was reported to have the decomposition rate of 2-propanol vapor by a photocatalytic reaction improved by about 1.2 times, as compared with a titanium dioxide layer formed on a silica-coated Pyrex glass.
However, the method or the sol-gel method has a problem such that it is difficult to obtain a uniform layer thickness over a large area such as a window glass, the surface smoothness of the layer is poor, or the abrasion resistance of the layer is inadequate.
The present invention has an object to provide a titanium dioxide photocatalyst carrier which exhibits a high photocatalytic activity and which has, as a surface layer, titanium dioxide excellent in the abrasion resistance.
Further, the present invention has an object to provide the above titanium dioxide carrier which exhibits a high photocatalytic activity and which is excellent also in the alkali resistance or the abrasion resistance.
Namely, the present invention provides a titanium dioxide photocatalyst carrier having a thin layer of titanium dioxide, which is formed by coating and heat-treating a titanium dioxide precursor solution prepared from a hydrolyzable titanium compound and an aromatic compound solvent, on a substrate surface.
Here, the change in the visible light transmittance as between before and after carrying out a Taber abrasion test on the surface of the thin layer of titanium dioxide by a truck wheel of CS-10F under such conditions as a load of 4.9N and 200 rotations, is preferably at most 5%.
Further, the present invention provides the titanium dioxide photocatalyst carrier which has a layer to prevent diffusion of an alkali component from the substrate to the thin layer of titanium dioxide, or a charge separation layer, between the substrate and the thin layer of titanium dioxide.
It is preferred that the geometrical thickness of the thin layer of titanium dioxide is from 5 to 1,000 nm, or the substrate is a glass substrate.
Further, it is preferred that the titanium dioxide precursor solution is one obtained by dissolving the hydrolyzable titanium compound in the aromatic compound solvent, followed by hydrolysis/dehydration condensation. The aromatic compound solvent is preferably at least one member selected from the group consisting of benzene, toluene, xylene and ethylbenzene. Further, the hydrolyzable titanium compound is preferably a titanium alkoxide.
Further, the present invention provides a process for producing a titanium dioxide photocatalyst carrier having a thin layer of titanium dioxide, which comprises coating and heat-treating a titanium dioxide precursor solution prepared from a hydrolyzable titanium compound and an aromatic compound solvent, on a substrate surface.